1. Field of the Technology
The present invention is generally directed to rotary cutting tools and rotary cutting tool blanks having a composite construction including regions of differing composition and/or microstructure, and to related methods. The present invention is more particularly directed to multi-grade cemented carbide rotary cutting tools and tool blanks for rotary cutting tools having a composite construction wherein at least one region comprises a hybrid cemented carbide including cubic carbide, and to methods of making the rotary cutting tools and rotary cutting tool blanks. The present invention finds general application to rotary cutting tools such as, for example, tools adapted for drilling, reaming, countersinking, counterboring, and end milling.
2. Description of the Background of the Technology
Cemented carbide rotary cutting tools (i.e., cutting tools driven to rotate) are commonly employed in machining operations such as, for example, drilling, reaming, countersinking, counterboring, end milling, and tapping. Such tools are conventionally manufactured with a non-hybrid solid monolithic construction. The manufacturing process for such tools involves consolidating metallurgical powder (comprised of particulate ceramic and metallic binder) to form a compact. The compact is then sintered to form a cylindrical tool blank having a monolithic construction. As used herein, the term “monolithic construction” means that a tool is composed of a solid material such as, for example, a cemented carbide, having substantially the same characteristics at any working volume within the tool. Subsequent to sintering, the tool blank is appropriately machined to form the cutting edge and other features of the particular geometry of the rotary cutting tool. Rotary cutting tools include, for example, drills, end mills, reamers, and taps.
Rotary cutting tools composed of cemented carbide are adapted to many industrial applications, including the cutting and shaping of materials of construction such as metals, wood, and plastics. Tools made of cemented carbide are industrially important because of the combination of tensile strength, wear resistance, and toughness that is characteristic of these materials. As is known in the art, cemented carbide is comprised of at least two phases: at least one hard ceramic component and a softer matrix of metallic binder. The hard ceramic component may be, for example, carbides of elements within groups IVB through VIB of the periodic table. A common example is tungsten carbide. The binder may be a metal or metal alloy, typically cobalt, nickel, iron, or alloys of these metals. The binder “cements” regions of the ceramic component within a matrix interconnected in three dimensions. Cemented carbides may be fabricated by consolidating a metallurgical powder blend of at least one powdered ceramic component and at least one powdered metallic binder.
The physical and chemical properties of cemented carbides depend in part on the individual components of the metallurgical powders used to produce the materials. The properties of a cemented carbide are determined by, for example, the chemical composition of the ceramic component, the particle size of the ceramic component, the chemical composition of the binder, and the ratio of binder to ceramic component. By varying the components and proportions of components in the metallurgical powder blend, cemented carbide rotary cutting tools such as drills and end mills can be produced with unique properties matched to specific applications.
The monolithic construction of rotary cutting tools inherently limits their performance and range of application. As an example, FIGS. 1(a) and 1(b) depict side and end views, respectively, of a twist drill 20 having a typical design used for creating and finishing holes in construction materials such as wood, metals, and plastics. The twist drill 20 includes a chisel edge 21, which makes the initial cut into the workpiece. The cutting tip 24 of the drill 20 follows the chisel edge 21 and removes most of the material as the hole is being drilled. The outer periphery 26 of the cutting tip 24 finishes the hole. During the cutting process, cutting speeds vary significantly from the center of the drill to the drill's outer periphery. This phenomenon is shown in FIGS. 2(a) and 2(b), which graphically compare cutting speeds at an inner (D1), outer (D3), and intermediate (D2) diameter on the cutting tip of a typical twist drill. In FIG. 2(a), the outer diameter (D3) is 1.00 inch and diameters D1 and D2 are 0.25 and 0.50 inch, respectively. FIG. 2(b) shows the cutting speeds at the three different diameters when the twist drill operates at 200 revolutions per minute. As illustrated in FIGS. 2(a) and (b), the cutting speeds measured at various points on the cutting edges of rotary cutting tools will increase with the distance from the axis of rotation of the tools.
Because of these variations in cutting speed, drills and other rotary cutting tools having a monolithic construction will not experience uniform wear at different points ranging from the center to the outside edge of the tool's cutting surface, and chipping and/or cracking of the tool's cutting edges may occur. Also, in drilling casehardened materials, the chisel edge is typically used to penetrate the case, while the remainder of the drill body removes material from the softer core of the casehardened material. Therefore, the chisel edge of conventional non-hybrid drills of monolithic construction used in drilling casehardened materials will wear at a much faster rate than the remainder of the cutting edge, resulting in a relatively short service life for such drills. In both instances, because of the monolithic construction of conventional non-hybrid cemented carbide drills, frequent regrinding of the cutting edge is necessary, thus placing a significant limitation on the service life of the drill. Frequent regrinding and tool changes also result in excessive downtime for the machine tool that is being used.
Other types of rotary cutting tools having a monolithic construction suffer from similar deficiencies. For example, specially designed drill bits often are used for performing multiple operations simultaneously. Examples of such drills include step drills and subland drills. Step drills are produced by grinding one or more steps on the diameter of the drill. Such drills are used for drilling holes of multiple diameters. Subland drills may be used to perform multiple operations such as drilling, countersinking, and/or counterboring. As with regular twist drills, the service life of step and subland drills of a conventional non-hybrid monolithic cemented carbide construction may be severely limited because of the vast differences in cutting speeds experienced at the drills' different cutting edge diameters.
The limitations of monolithic rotary cutting tools are also exemplified in end mills. In general, end milling is considered an inefficient metal removal technique because the end of the cutter is not supported, and the length-to-diameter ratio of end mills is usually large (usually greater than 2:1). This causes excessive bending of the end mill and places a severe limitation on the depths of cut and feed rates that can be employed.
In order to address the problems associated with rotary cutting tools of a monolithic construction, attempts have been made to produce rotary cutting tools having different properties at different locations. For example, cemented carbide drills having a decarburized surface are described in U.S. Pat. Nos. 5,609,447 and 5,628,837. In the methods disclosed in those patents, carbide drills of a monolithic cemented carbide construction are heated to between 600-1100° C. in a protective environment. This method of producing hardened drills has major limitations. First, the hardened surface layer of the drills is extremely thin and may wear away fairly quickly to expose the underlying softer cemented carbide. Second, once the drills are redressed, the hardened surface layer is completely lost. Third, the decarburization step, which is an additional processing step, significantly increases the cost of the finished drill.
The limitations associated with monolithic cemented carbide rotary cutting tools have been alleviated by employing a “composite” construction, as described in U.S. Pat. No. 6,511,265 (“the '265 patent”), which is incorporated herein by reference in its entirety. The '265 patent discloses a composite rotary cutting tool including at least a first region and a second region. The tool of the '265 patent may be fabricated from cemented carbide, in which case a first region of the composite rotary cutting tool comprises a first cemented carbide that is autogenously bonded to a second region of the tool, which comprises a second cemented carbide. The first cemented carbide and the second cemented carbide differ with respect to at least one characteristic. The characteristic may be, for example, modulus of elasticity, hardness, wear resistance, fracture toughness, tensile strength, corrosion resistance, coefficient of thermal expansion, or coefficient of thermal conductivity. The regions of cemented carbide within the tool may be coaxially disposed or otherwise arranged so as to advantageously position the regions to take advantage of their particular properties.
While the invention described in the '265 patent addresses certain limitations of monolithic cemented carbide rotary cutting tools, the examples of the '265 patent primarily contain tungsten carbide. Since relatively high shear stresses are typically encountered in rotary cutting tools employed for drilling, end-milling, and similar applications, it is advantageous to employ cemented carbide grades having very high levels of strength, such as those employing tungsten carbide. Those grades, however, may not be suitable for machining steel alloys due to a reaction that can occur between iron in the steel workpiece and tungsten carbide in the rotary cutting tool. Tools used for machining steels may contain 0.5% or more cubic carbides in a monolithic conventional grade cemented carbide. The addition of cubic carbides in such tools, however, generally results in a decrease in tool strength.
Thus, there exists a need for drills and other rotary cutting tools having different characteristics at different regions of the tool, such as high strength and hardness, and which do not chemically react with the workpiece.